Subtyping

As explained in Functional Programming in Scala, Second Edition

Subtyping is a type-system relationship in which values of one type may be used where values of another, more general type are expected.

How Functional Programming in Scala, Second Edition covers it

How deep it goes

Descends from a motivating type error into covariance, contravariant positions, and mechanically widened type parameters.

Examples

Uses a cat program that reads lines from Files and prints them through Console.

What it assumes

Assumes Scala type parameters and Free-program syntax while developing variance concerns through compiler errors.

What you can do after

Build it from scratch by widening Free effect types and correcting flatMap variance errors.

How it connects

Connects subtyping with covariance, variance, union types, flatMap, and Free programs.

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Also explained in

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  100 SQL Server Mistakes and How to Avoid ThemA subtype is a specialized entity type that inherits common attributes from a supertype and may add unique attributes.
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  Kotlin in Action, 2EA subtyping relation specifies when one type can be used wherever another type is expected.
• 

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  Julia as a Second LanguageA subtype is a type whose values are also accepted wherever values of a more general supertype are expected.

Compare how 4 books explain subtyping →

Related concepts

• superclasscontrasts with · A superclass is a class from which another class inherits members, structure, state, or behavior.
• supertypeimplements · A supertype is a type that generalizes one or more more-specific subtypes and encompasses values assignable to those subtypes.
• type compatibilitymeasures · Type compatibility is the relationship in which values of one type can be used where another type is expected under applicable language rules.
• attributeconfigures · An attribute is a named property, value, or piece of information associated with an entity, object, or program element.

This explainer was generated by Manning's knowledge graph from the text of Functional Programming in Scala, Second Edition. The section links point to the parts of the book it draws on.